The present invention relates generally to inkjet printing, and, more particularly, to stabilizing colorants printed onto an ink-receiving layer applied to a non-absorbant substrate.
1. Inkjet Receiving Layers
Inkjet receiving layers need to absorb the ink vehicle delivered during the printing process. When the ink-receiving layer is applied to non-absorbent substrate, the substrate provides no absorption capacity and as a result, the ink-receiving layer must be the sole absorbing material. To increase the absorbing capacity of the coating, an absorbent basecoat has been described in the prior art that serves to increase the capacity of the coating, much as a substrate functions in paper-based inkjet media.
A topcoat is applied to control surface properties such as gloss, tackiness, surface energy, and durability, as well as to function in concert with the adsorbent basecoat. In addition, the topcoat must be free of defects that would contribute to perceived irregularities or non-uniformities in the coating.
U.S. Pat. No. 5,275,867 describes a two-layer coating and a coating process where a topcoat is laminated on the basecoat. U.S. Pat. No. 5,605,750 describes a three-layer coating and a coating process where the topcoats are applied to the precoat by coating both fluids before drying in a multi-slot hopper or a slide hopper. U.S. Pat. No. 5,576,088 describes a two layer coating and a coating process where a topcoat is cast coated on a basecoat. All these examples describe a process that involves specialized equipment and coatings engineered to be compatible with the processes. In addition, production efficiencies may be lower.
An on-going problem in the application of a topcoat with basic coating equipment such as mayer rod and slot die coaters is the formation of bubbles in the topcoat when it is coated on a porous basecoat that has been applied to a non-porous substrate. These bubbles are formed when the air voids in the pores of the precoat are filled with fluid from the topcoat application process which results in the air being forced to surface of the precoat where they coalesce into bubbles in a still fluid topcoat. These bubbles can then form defects in the topcoat as that coating is dried. Another challenge when developing coating fluids and chemistries is avoiding problems associated with incompatible chemistries that result in solution gelling or phase separation in the dried coatings.
Above-referenced application Ser. No. 09/491,642 provides a solution to the foregoing problems by adding a re-wet liquid coating subsequent to forming the basecoat and prior to forming the topcoat. The resulting process permits the topcoat to be applied to a porous basecoat that is coated on a non-porous, or non-permeable, substrate such that bubbles are not formed in the topcoat. This allows the production of defect-free coatings.
2. Stabilization of Colorant
Porous inkjet media provides superior image quality and essentially instant drying, which permits high throughput in inkjet printing. However, because of the porosity, dyes residing on the porous media can be easily accessed by oxygen and moisture. The dyes have very poor stability when exposed to light, especially ultraviolet (UV) light.
A light-fast dye printed on well-designed swellable media can have a lifetime of five or more years; however, such a dye may last only few weeks, or even only a few days, on porous media. It is important to improve the light-fastness of inkjet ink on porous media to obtain the many positive attributes of porous media.
To the best of the inventors"" knowledge, there is no good solution yet existing in this field. Presently-available commercial porous media without lamination or post-printing treatment can only last three months in office lighting because of image fade.
The most common way to improve light-fastness is through the use of light-fast dyes or pigments (colorants). The majority of light-fast dyes are metal complex dyes. There are often toxicity concerns with such metal complex dyes. In addition, these metal complex dyes are dull in color. Good color reproduction and sharp tones are not achievable with those dyes.
Although pigmented inks provide good light/oxygen resistance, they do not render the high image quality of a dye-based ink.
Many light-fastness additives have been tested in both media formulations and ink formulations. Some of these additives are well-reported UV absorbers, radical scavenger, and antioxidants. These additives have been shown to be effective for non-porous media. However, many of these additives are not effective for porous glossy media. Some of these additives have shown a slight improvement in light-fastness, but caused gloss loss and media formulation instability (i.e., non-manufacturable).
An example of a reference disclosing light-fastness additives is U.S. Pat. No. 5,855,655, entitled xe2x80x9cColorant Stabilizersxe2x80x9d, issued to R. S. Nohr et al on Jan. 5, 1999, and assigned on its face to Kimberly-Clark Worldwide, Inc. This reference discloses a number of colorant stabilizers, including (1) certain aryliminealkenes, (2) heavy metal ions, such as iodide ions, (3) certain derivatives of phenols, (4) reducing agents, such as sodium thiosulfate, sodium sulfite, cysteine, sodium nitrite, sodium phosphite, and citric acid, (5) certain molecular includants, such as clathrates, zeolites, crown ethers, calixarenes, valinomycin-type natural antibiotics, various polyether compounds, nigericin-type natural antibiotics, and cyclic compounds containing a plurality of pyranose rings such as cyclodextrins, (6) certain porphines, (7) metals or metal salts alone or in combination with at lest one other colorant stabilizer, wherein the metal or metal salt is a lanthanide element or salt, respectively, magnesium, iron, zinc, other transition metals, and heavy metals, (8) dimethyl amino benzoic acid quat, (9) a basic fuschin hydrazone, and (10) a benzophenone. The colorant stabilizer may be added to (1) solvent- or oil-based colorant compositions or (2) water-based colorant compositions or may be formed on the surface of a print medium, such as inkjet paper. Examples of print media listed include a wood product or composite, woven fabric, nonwoven fabric, textile, plastic, and glass. Examples of plastic substrates include plastic films, plastic nonwoven webs, and plastic woven webs. The preferred substrate is paper, including printing and writing papers, packaging and industrial papers, paperboard, and tissue papers.
While many specific examples of papers are listed, there is no specific mention of glossy paper, such as used in photographic-quality reproduction. Further, it turns out that glossy paper may not simply be coated with one of the above-listed colorant stabilizers, due to chemistry considerations, namely, the possible loss of gloss, the possible adverse impact on print quality, the possible drift in pH, and the possible agglomeration of pigment (colorant) and/or polymer.
Thus, there remains a need for a method that provides colorant stabilization for glossy print media.
In accordance with the present invention, at least one of the following compounds is applied as a re-wet solution to media: potassium iodide, sodium thiosulfate, and sodium thiocyanate. The addition of at least one of these three compounds to the re-wet solution can improve light fastness on the order of eight to thirteen times on porous glossy media. These additives are particularly effective with dyes such as Reactive Black 31 (Pacified) (RB31), Direct Blue 199 (DB199), Magenta 377 (M377), and Direct Yellow 132 (DY132).
In particular, an improved process is provided for applying at least one ink-receiving layer to a non-permeable substrate. The ink-receiving layer is used to stabilize at least one colorant in at least one ink printed thereon. The ink-receiving layer comprises a non-permeable substrate, a porous basecoat formed thereon and comprising a plurality of pores, and a topcoat formed on said porous base coat. The process comprises:
(a) applying the porous basecoat to a surface of the non-permeable substrate;
(b) applying a re-wet liquid to the porous basecoat; and
(b) applying a topcoat on the re-wet liquid.
The improvement comprises formulating said re-wet liquid to contain at least one compound selected from the group consisting of potassium iodide, sodium thiosulfate, and sodium thiocyanate.